Date of Award

Summer 2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical & Computer Engineering

Program/Concentration

Electrical Engineering

Committee Director

Gon Namkoong

Committee Member

Hani E. Elsayed-Ali

Committee Member

Helmut Baumgart

Committee Member

Tarek Abdel-Fattah

Abstract

Organic-inorganic halide perovskite solar cells have emerged as a promising photovoltaic technology due to their superb power conversion efficiency (PCE) and very low material costs. While perovskite solar cells are expected to eventually compete with existing silicon-based solar cells on the market, their long-term stability has become a major bottleneck. In particular, perovskite films are found to be very sensitive to external factors such as air, UV light, light soaking, thermal stress and others. Among these stressors, light, oxygen and moisture-induced degradation can be slowed by integrating barrier or interface layers within the device architecture. However, the most representative perovskite absorber material, CH3NH3PbI3 (MAPbI3), appears to be thermally unstable even in an inert environment. This poses a substantial challenge for solar cell applications because device temperatures can be over 45 °C higher than ambient temperatures when operating under direct sunlight. In this thesis, the thermal stability of perovskite solar cells was primarily investigated.

Initially, we systematically studied the effects of heating and cooling processes on the principal photovoltaic performance of perovskite solar cells by combining temperature-dependent J-V, steady-state PL, UV-VIS and time-resolved lifetime decay measurements. In particular, we have observed the dynamic evolution of degraded crystallinity, increased charge trapping, deep trap depth and PbI2 phase. During the heating process, the thermal degradation of the perovskite film was observed at 70 ° C or higher. An increase in the disordered phase of the perovskite film involved a drastic increase in charge trapping and the development of a deeper trap depth. Interestingly, we observed that the degradation of the perovskite film persisted even after the temperature was dropped, which led to irreversible J-V characteristics of the perovskite solar cell.

Later, we introduced a polymer layer of PMMA which improved thermal stability for more than 1000hrs at 85°C. Without PMMA, host-casted MAPbI3 films suffered rapid thermal degradation, forming a number of pin-holes at GBs and then extending into GIs. Rapid thermal degradation of perovskite GBs without PMMA may be due to the rich moisture chemical structure of hydrated (CH3NH3)4PbI6•H2O. At the elevated temperature, hydrated (CH3NH3)4PbI6•H2O grain boundaries might suffer from moisture-assisted decomposition, forming a number of pin-holes at GBs. Conversely, we observed high thermal stability of perovskite films by introducing PMMA to induce marked thermal stability at GBs. It is believed that the excellent hygroscopicity of PMMA played an active role in absorbing moisture from hydrated (CH3NH3)4PbI6•H2O GBs and driving them out through the GB channel. We believe that continuous functionalization of perovskite GBs or crosslinking perovskite GBs with PMMA molecules might drastically render perovskite GBs chemically robust, resilient, and heat-resistant. Moreover, we mixed inorganic cesium (Cs) cation into the perovskite, which improved thermal stability at a higher temperature of 120°C.

Finally, we have fabricated perovskite solar cells in an antisolvent method in which the perovskite film does not contain deeper grain boundary like hot-casted perovskite thin film. Also, we introduced a polymer (polyimide) on the top of the perovskite solar cell which has a large contact angle and glass transition temperature. Consequently, perovskite solar cells with polyimide showed thermal stability without any efficiency decrement more than 30 days.

DOI

10.25777/x3e3-5x18

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